1. Field
The present disclosure relates to techniques for modulating optical signals. More specifically, the present disclosure relates to an electro-refraction modulator with a single-crystal regrown p-n junction.
2. Related Art
Silicon photonics is a promising technology that can provide large communication bandwidth, low latency and low power consumption for inter-chip and intra-chip optical interconnects or links. A key component for use in inter-chip and intra-chip optical interconnects is a modulator that can be monolithically integrated into the same silicon layer as transistors and other optical components.
Existing modulator architectures, such as a ring-resonator modulator and Mach-Zehnder interferometer (MZI) modulator, typically use interference and phase control to achieve the modulation effect. In particular, the tuning efficiency of a ring-resonator modulator (in pm/V) or the required length (L) of the MZI modulator (such as the product of L needed for a 180° phase shift) are usually important considerations that relate to the energy efficiency and the size of the modulators, thus helping to define the integration potential of the modulators. In general, the energy efficiency and the size are directly related to the design of the phase-shifting section of a modulator.
Because electro-optical (EO) effects in single-crystal silicon are often relatively weak (compared to, for example, those in gallium arsenide and lithium-niobate), historically phase shifting in silicon was accomplished either by a thermo-optic effect (such as by integrating micro-heating elements in or proximate to a modulator) and/or by a free-carrier dispersion effect (e.g., by integrating a forward-biased p-i-n junction in a modulator). However, both of these approaches are inadequate for high-speed modulators, because of their limited bandwidth (typically in kiloHertz range for thermal tuning and up to a few gigaHertz for carrier injection, unless CMOS-unfriendly pre-emphasized signals are used at a cost of more energy per bit).
Consequently, photonic designers often use reverse-biased p-n junctions as the phase-shifting elements. These devices use the widening of the depletion zone of the p-n junction as a function of the applied reverse bias to modify the effective index of refraction of the optical waveguide in a modulator and, thus, to control the phase. Calculated initial and modulated widths of a p-n junction depletion zone as a function of the doping level (with equal donor and acceptor species) and the reverse bias voltage are summarized in Table 1.
TABLE 1Doping levelDepletion regionDepletion regionDepletion region(cm−3)width (nm) at 0 Vwidth (nm) at −1 Vwidth (nm) at −2 V1 · 1017145.1 216.6 273.3 5 · 101770.0100.6 124.0 1 · 101849.170.788.25 · 101823.432.840.11 · 101916.523.028.5
Using reverse-biased p-n junctions, modulators with bandwidths greater than 50 GHz have been obtained. However, the phase change achievable using a reverse-biased p-n junction is typically smaller than that obtainable using thermal tuning or carrier injection in comparably sized modulator. Consequently, MZI modulators that include reverse-biased p-n junctions usually require several millimeters of length in order to achieve a 180° phase change.
Similarly, ring-resonator modulators with reverse-biased p-n junctions would also benefit from increased tuning efficiency. Table 2 provides calculated phase-tuning efficiency and absorption-induced loss of lateral (or horizontal) and vertical p-n junctions as a function of the doping level (with equal donor and acceptor species). These calculations use a 300 nm thick silicon-on-insulator technology, with a 220 nm etch depth for optical waveguide definition and a 380 nm wide, single-mode optical waveguide.
TABLE 2Lateral junctionVertical junctionAbsorption-inducedDoping leveltuning efficiency tuning efficiencypropagation loss(cm−3)(pm/V)(pm/V)(dB/cm)1 · 1017 7.9910.45 0.385 · 101717.9856.58 2.641 · 101843.8785.6516.605 · 101882.23119.25 96.311 · 1019109.38 191.41 416.87 
As shown in Table 2, the achievable tuning efficiency increases monotonously and sharply (roughly exponentially) with increased doping levels. However, higher doping levels result in a significant increase in the absorption-induced optical waveguide propagation loss. Therefore, for practical applications, designers typically limit the doping to less than 2·1018/cm3, which, as noted previously, results in millimeter-length MZI modulators. Moreover, ring-resonator modulators usually sacrifice the Q-factor for increased tuning efficiency because of the increased propagation loss.
Furthermore, the limited tuning efficiencies of existing modulators often result in the use of higher drive voltages. For example, existing modulators typically use drive voltages of more than 2 V. As CMOS technology nodes scale to smaller critical dimensions (and, thus, to higher optical data rates), the drive voltage decreases, which can reduce the performance of modulators.
Hence, what is needed is a modulator without the above-described problems.